I’ll reply to each point made by you individually.
Firstly, the use of third person here does not constitute as third person abuse. While the mugger may have used it gain an edge on you, this is not third person abuse. Third person abuse would be using it to see through solid objects you would not normally be able to do, such as walls to see inside of a building. As for the PK appeal that an uninvolved member graciously posted, that PK was overturned for reasons beyond the use of third person despite not being detailed in the reply.
Secondly, while the doorway is SLIGHTLY in view of the NPCs line of sight, I believe it is far way enough and not in its direct view for it to not be an issue. Regardless most of the mugging takes place while you are inside the building/doorway.
Lastly frankly I don’t believe it’s too much of a coincidence for someone to be waiting to mug someone who comes into a back alley & back door of a building. Kind of a prime/realistic mugging spot honestly.